Currently available popcorn intended for cooking in a microwave oven is usually packaged in a two-ply paper bag, which may have a flat bottom or a wedge-shaped bottom. The bag contains a charge of edible ingredients, usually including corn kernels, shortening and sometimes seasoning or flavoring.
The bags are first formed in a bag-making plant and are later filled as part of a separate operation, usually at a different location. The bags should therefore be readily stackable, shipable and be handleable in a mechanized manner.
Since the volume of popped corn is vastly greater than that of the unpopped kernels, the container must have the ability to expand during cooking. For this reason, the bag is capable of unfolding under the internal pressure of the steam released by the exploding kernels and the pressure of the expanding kernels themselves. Bags of this type are described in U.S. Pat. Nos. 4,571,337 (Cage et al.) and 3,851,574 (Katz et al.). Although these patents show bags with flat bottoms, bags with wedge-shaped bottoms are presently more common.
The configuration and construction of the bag should help to maximize the "pop volume" of the corn. Not only does increased pop volume result in a larger volume of edible product (both real and perceived), but it has been found that increased pop volume results in a lighter, fluffier popcorn product with improved textural qualities.
In general, bags with wedge-shaped bottoms containing ready-to-pop corn are of either pinched bottom or tube construction. These two types of bags are similar in most respects, the bottom being formed by bonding together the front and back panels of the bag at their bottom edges. There is thus no separate bottom panel. A typical bag may include a seamless and generally flat front panel, a generally flat back panel with an overlapping seam running its full length at or near its center, and two side panels that connect the front and back panels. The side panels are each folded inwardly to form a gusset that permits the bag to expand. In a pinched bottom bag the sealed bottom edge is folded over itself, whereas in a tube bag there is no such fold at the bottom.
In a typical ready-to-pop bag the edible charge is placed inside the bag, resting on a susceptor that is contiguous with a center section of the front panel. The bag is sealed at the top after filling by bonding together the top edges of the front and back panels. The gussets of the folded bag intrude well into the bag interior, extending between at least some portion of the charge and the back panel. Since the upper and lower sections of the bag remain essentially empty, they are easily flattened and folded over the more bulky center section.
When the still folded bag is to be used, it is placed inside the oven with the center section of the front panel resting on the oven floor, so that the charge rests on the susceptor and the gussets of the side panels overlay a portion of the charge. During cooking, the internal pressure of the steam released by the kernels and the pressure of the popped kernels themselves cause the two end sections of the bag to unfold gradually.
A typical popcorn bag may, however, yield less than the maximum possible pop volume. This common deficiency in bag performance can be attributable, at least in part, to the configuration of the bag and to such factors as the size and stiffness of the gussets.
The gussets intrude into the bag to the greatest extent when the bag is folded flat and the gussets are closed. As the cooking commences, the gussets define a relatively narrow channel between them, which widens as the cooking progresses and the bag expands. The popped kernels must pass through this channel to move vertically into the upper section of the bag. Consequently some of the kernels that are among the first to pop are forced to move laterally a considerable distance toward the center of the bag before they can move upwardly away from the susceptor.
In addition, some kernels break loose from the charge prior to or during cooking and lodge themselves in the folds between the panels and the gussets. In many instances the gussets do not open sufficiently or early enough to release these kernels and allow them to pop properly.
The opening of the gussets as the bag expands is essential to freeing the trapped kernels, but does not, in itself, assure that these kernels return to the susceptor and the center of the bag. The loose kernels are, however, prompted to so move by any rounding of the front panel (on which the bag rests during cooking) that takes place and by the shaking movement of the bag caused by the exploding kernels. The more pronounced the curvature of the front panel, the more tendency the kernels will have to move toward the susceptor.